1. Field of the Invention
The invention relates to the production of aluminum by igneous electrolysis. More particularly, the present invention relates to means of confining gaseous effluents produced during electrolysis.
2. Description of Related Art
Metal aluminum is produced industrially by igneous electrolysis, namely by electrolysis of alumina in solution in a molten cryolite bath called an electrolyte bath, using the well-known “Hall-Héroult Process.” The electrolyte bath is contained in pots called “electrolytic pots”, comprising a steel pot shell lined on the inside with refractory and/or insulating materials, and a cathode assembly located at the bottom of the pot. Anodes made of a carbonaceous material are partially immersed in the electrolyte bath. The assembly including an electrolytic pot, its anodes and the electrolyte bath is typically referrer to as an electrolytic cell.
The electrolysis reaction, secondary reactions and high operating temperatures cause the production of gaseous effluents that, in particular, comprise carbon dioxide and products containing fluorine. Release of these effluents into the atmosphere is severely controlled and governed by strict regulations, firstly for the ambient atmosphere in the electrolysis room in consideration of the working conditions of personnel close to the cells, and secondly for atmospheric pollution. Regulations in several countries and states impose pollution limits on effluent quantities released into the atmosphere.
At the moment, solutions for extracting, recuperating and treating these effluents in a reliable and satisfactory manner have been proposed. One widespread solution involves providing the electrolytic cells with an effluent collection device. This device covers the electrolytic pots and comprises confinement means that in particular include a covering device, as well as a means of suction and chemical treatment of the effluents. Known processes for the treatment of effluents include particularly recuperation of gases containing fluorine by reaction with alumina. The covering device comprises access means such as hoods, usually removable, and a tapping door used to work on the pot.
The covering device delimits a confined suction area at a negative pressure relative to the ambient atmosphere, so that effluents can be recovered efficiently. The result is that collection efficiencies under steady state conditions sometimes exceed 97% in most modern industrial installations, such that release of gaseous products containing fluorine into the atmosphere are significantly lower than regulatory limits.
In general, anodes are connected to an electric power supply bar located outside the collection device through metallic stems that pass through the device by openings formed therein. The free space (or “clearance”) left by stems in these openings is not sealed, to enable vertical and horizontal displacements of the metallic stems. Vertical displacements are frequent, and in particular, compensate for wear of anodes during electrolysis. Horizontal displacements generally occur during operations to replace used anodes.
Free spaces between anode stems and the inside edge of passage openings form a confinement break that while being relatively insignificant for each individual anode stem, becomes significant when considering all anodes in a pot, and even more significant for a series of several hundred cells.